Referring to FIG. 1, the conventional rotary gear pump 1 has a pump casing 2 and also has a driving gear 3 and a driven gear 4 both arranged inside the pump casing 2. When the driving gear 3 rotates in one direction, the driven gear 4 is driven to rotate in the opposite direction. Thereby, a fluid is driven to flow in the space between the teeth and then pumped out from an output end 5.
The tooth profile of a gear can be generated by a rack. Refer to FIG. 2 for two involute gears 6 meshing with each other inside the rotary gear pump 1. The tooth profile of the involute gear 6 including an involute 8, a root fillet 9 and a rounded tip 10.
When the two involute gears 6 mesh to rotate, the contact points 7 thereof form a contact path 11. As shown in FIG. 2, the contact path 11 is an unsmooth continuous curve including two straight lines and two arcs. The straight lines are generated by the contacting of the involutes 8 of the two involute gears 6. The arcs are generated by the contacting of the rounded tip 10 and root fillet 9 of the two involute gears 6. In the contact path 11, the tangent of the arc and the tangent of the straight line are not continuous. Therefore, the contact path 11 is not a smooth continuous curve. It means that the first-order derivatives of the contact path 11, i.e. the tangents thereof, are discontinuous.
For a rotary gear pump 1, a discontinuous contact path 11 of the involute gears 6 results in rippled pressure in the outlet end 5 and causes noise in operation. Thus, the rotary gear pump 1 hardly can achieve a precise and stable output and it is unlikely to meet the requirement of high-precision, high-stability and high-quality products, such as medical pumps.
Most of the rotary gear pumps 1 are used to pump fluids, especially fluids almost without compressibility. Refer to FIG. 3 for the meshing state of the two gears shown in FIG. 2. An encapsulation 14 may form between the addendum 12 and the dedendum 13 when two point-contacts occur in some moments during the meshing of the two involute gears 6. The fluid trapped in the encapsulation 14 is not allowed to leave. Thus, the pressurized incompressible fluid may damage the machine and generate noise.